android - flutter 错误 "the method ' toLowerCase( )' was called on null"

Question

我正在 flutter 中创建一个 Skype 克隆应用程序，但在执行“搜索”功能时遇到错误。我想通过一个在您键入时更新的列表，通过以小写形式返回的用户名或名称在我的 firebase 数据库中搜索其他用户。但是，在第一次击键时我遇到了这个错误:

The method 'toLowerCase' was called on null.
Receiver: null
Tried calling: toLowerCase()

这是搜索屏幕的代码(问题在 buildSuggestions 下):

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:gradient_app_bar/gradient_app_bar.dart';
import 'package:skypeclone1/models/user.dart';
import 'package:skypeclone1/resources/firebase_repository.dart';
import 'package:skypeclone1/utils/universal_variables.dart';
import 'package:skypeclone1/widgets/custom_tile.dart';

class SearchScreen extends StatefulWidget {
  @override
  _SearchScreenState createState() => _SearchScreenState();
}

class _SearchScreenState extends State<SearchScreen> {
  FirebaseRepository _repository = FirebaseRepository();

  List<User> userList;
  String query = "";
  TextEditingController searchController = TextEditingController();

  @override
  void initState() {
    // TODO: implement initState
    super.initState();

    _repository.getCurrentUser().then((FirebaseUser user) {
      _repository.fetchAllUsers(user).then((List<User> list) {
        setState(() {
          userList = list;
        });
      });
    });
  }

  searchAppBar(BuildContext context) {
    return GradientAppBar(
      backgroundColorStart: UniversalVariables.gradientColorStart,
      backgroundColorEnd: UniversalVariables.gradientColorEnd,
      leading: IconButton(
        icon: Icon(Icons.arrow_back, color: Colors.white),
        onPressed: () => Navigator.pop(context),
      ),
      elevation: 0,
      bottom: PreferredSize(
        preferredSize: const Size.fromHeight(kToolbarHeight + 20),
        child: Padding(
          padding: EdgeInsets.only(left: 20),
          child: TextField(
            controller: searchController,
            onChanged: (val) {
              setState(() {
                query = val;
              });
            },
            cursorColor: UniversalVariables.blackColor,
            autofocus: true,
            style: TextStyle(
              fontWeight: FontWeight.bold,
              color: Colors.white,
              fontSize: 35,
            ),
            decoration: InputDecoration(
              suffixIcon: IconButton(
                icon: Icon(Icons.close, color: Colors.white),
                onPressed: () {
                  WidgetsBinding.instance
                      .addPostFrameCallback((_) => searchController.clear());
                },
              ),
              border: InputBorder.none,
              hintText: "Search",
              hintStyle: TextStyle(
                fontWeight: FontWeight.bold,
                fontSize: 35,
                color: Color(0x88ffffff),
              ),
            ),
          ),
        ),
      ),
    );
  }

  buildSuggestions(String query) {
    final List<User> suggestionList = query.isEmpty
        ? []
        : userList.where((User user) {
            String _getUsername = user.username.toLowerCase();
            String _query = query.toLowerCase();
            String _getName = user.name.toLowerCase();
            bool matchesUsername = _getUsername.contains(_query);
            bool matchesName = _getName.contains(_query);

            return (matchesUsername || matchesName);


          }).toList();

    return ListView.builder(
      itemCount: suggestionList.length,
      itemBuilder: ((context, index) {
        User searchedUser = User(
            uid: suggestionList[index].uid,
            profilePhoto: suggestionList[index].profilePhoto,
            name: suggestionList[index].name,
            username: suggestionList[index].username);

        return CustomTile(
          mini: false,
          onTap: () {},
          leading: CircleAvatar(
            backgroundImage: NetworkImage(searchedUser.profilePhoto),
            backgroundColor: Colors.grey,
          ),
          title: Text(
            searchedUser.username,
            style: TextStyle(
              color: Colors.white,
              fontWeight: FontWeight.bold,
            ),
          ),
          subtitle: Text(
            searchedUser.name,
            style: TextStyle(color: UniversalVariables.greyColor),
          ),
        );
      }),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: UniversalVariables.blackColor,
      appBar: searchAppBar(context),
      body: Container(
        padding: EdgeInsets.symmetric(horizontal: 20),
        child: buildSuggestions(query),
      ),
    );
  }
}

如果我从 buildSuggestions 中省略 toLowerCase() 小部件，我会收到类似的错误:

The method 'contains' was called on null.
Receiver: null
Tried calling: contains("h")

*h 是我的击键

Answer 1

你的构建函数应该是这样的，它只在 userList 不为空时运行 buildSuggestions。

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: UniversalVariables.blackColor,
      appBar: searchAppBar(context),
      body: Container(
        padding: EdgeInsets.symmetric(horizontal: 20),
        child: userList.isEmpty?Container():buildSuggestions(query),
      ),
    );
  }